Lighting device with light sources positioned near the bottom surface of a waveguide

ABSTRACT

A device according to embodiments of the invention includes a waveguide, typically formed from a first section of transparent material. A light source is disposed proximate a bottom surface of the waveguide. The light source comprises a semiconductor light emitting diode and a second section of transparent material disposed between the semiconductor light emitting diode and the waveguide. Sidewalls of the second section of transparent material are reflective. A surface to be illuminated is disposed proximate a top surface of the waveguide. In some embodiments, an edge of the waveguide is curved.

This application is a continuation of U.S. application Ser. No.12/503,915, filed on Jul. 16, 2009. U.S. Ser. No. 12/503,915 isincorporated herein by reference.

FIELD OF INVENTION

The present invention is directed to lighting devices includingsemiconductor light emitting diodes.

BACKGROUND

Semiconductor light emitting devices such as light emitting diodes(LEDs) are among the most efficient light sources currently available.Material systems currently of interest in the manufacture of highbrightness LEDs capable of operation across the visible spectrum includegroup III-V semiconductors, particularly binary, ternary, and quaternaryalloys of gallium, aluminum, indium, and nitrogen, also referred to asIII-nitride materials; and binary, ternary, and quaternary alloys ofgallium, aluminum, indium, arsenic, and phosphorus. Often III-nitridedevices are epitaxially grown on sapphire, silicon carbide, orIII-nitride substrates and III-phosphide devices are epitaxially grownon gallium arsenide by metal organic chemical vapor deposition (MOCVD),molecular beam epitaxy (MBE), or other epitaxial techniques. Often, ann-type region is deposited on the substrate, then a light emitting oractive region is deposited on the n-type region, then a p-type region isdeposited on the active region. The order of the layers may be reversedsuch that the p-type region is adjacent to the substrate.

One promising use of semiconductor light emitting devices is forbacklights for general illumination and display devices such as liquidcrystal displays (LCDs). Color or monochrome transmissive LCDs arecommonly used in cellular phones, personal digital assistants, portablemusic players, laptop computers, desktop monitors, and televisionapplications.

One example of a backlight where light is provided by LEDs isillustrated in FIG. 5 is described in U.S. Pat. No. 7,052,152. An arrayof LEDs 43 is placed on the rear panel of the backlight 45. The backplane 48 and sidewalls 46 of the backlight 45 are covered with highlyreflective materials. A color converting phosphor layer 47 is disposedon a cover plate 40 of backlight 45. LCD panel 44 is placed in front ofbacklight 45. LCD panel 44 may be a conventional LCD, having a firstpolarizing filter, a thin film transistor array for developing anelectric field across selected areas of the liquid crystal layer, aliquid crystal layer, an RGB color filter array, and a second polarizingfilter. The color filter array has red, green and blue subpixels.Between the LCD panel 44 and the backlight 45, additional films areoften used, such as a brightness enhancement film (BEF) or polarizationrecovery film (DBEF).

SUMMARY

It is an object of the invention to form a device with a light sourcedisposed on the bottom surface of a solid, transparent waveguide. Adevice according to embodiments of the invention includes a waveguide,typically formed from a first section of transparent material. A lightsource is disposed proximate a bottom surface of the waveguide. Thelight source comprises a semiconductor light emitting diode and a secondsection of transparent material disposed between the semiconductor lightemitting diode and the waveguide. Sidewalls of the second section oftransparent material are reflective. A surface to be illuminated isdisposed proximate a top surface of the waveguide. In some embodiments,an edge of the waveguide is curved.

Lighting devices according to embodiments of the invention may bethinner than conventional devices, with sufficient illumination, mixing,and uniformity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an illumination system according to embodiments ofthe invention.

FIGS. 2 and 3 illustrate semiconductor light emitting devices connectedto the bottom of waveguides.

FIG. 4 is a top view of a portion of a waveguide.

FIG. 5 is a cross sectional view of a backlight and an LCD.

DETAILED DESCRIPTION

The waveguide illustrated in FIG. 5, which is formed by back plate 48,sidewalls 46 and cover plate 40, must be thick, in order for the lightincident on LCD 44 to be sufficiently mixed and uniform. Instead of anopen, box-like waveguide as illustrated in FIG. 5, in embodiments of theinvention, a solid waveguide is used. Light sources are positionedadjacent a bottom surface of the waveguide. Lighting devices accordingto embodiments of the invention may be thinner than the deviceillustrated in FIG. 5.

FIG. 1 illustrates a lighting device according to embodiments of theinvention. Several light sources 8 are coupled to the bottom surface ofwaveguide 6. Waveguide 6 may be, for example, a section of transparentmaterial that mixes light provided by several light sources. Waveguide 6may be, for example, acrylic (e.g., PMMA), hard silicone, moldedplastic, polycarbonate, or any other suitable material. Light fromwaveguide 6 is directed toward a surface to be illuminated. Though theembodiments below use the example of a liquid crystal display (LCD)panel 4 as the surface to be illuminated, the invention is not limitedto LCD displays. The surface to be illuminated may be any surfaceincluding, in the case of a general lighting application, a simpletransparent cover.

The surface to be illuminated may be a conventional LCD 4 having a firstpolarizing filter, a thin film transistor array for developing anelectric field across selected areas of the liquid crystal layer, aliquid crystal layer, an RGB color filter array, and a second polarizingfilter. The color filter array has red, green and blue subpixels.Between the LCD panel 4 and the waveguide 6, additional well-known filmscan be used, such as a brightness enhancement film or polarizationrecovery film, as well as a diffuser element to improve uniformity.

FIG. 2 illustrates a first example of a light source 8 which is coupledto a bottom surface of waveguide 6. A semiconductor LED such as a blue-or UV-emitting III-nitride LED 12 is connected by interconnects 14 to amount 10. LED 12 may be, for example, a thin-film flip-chip device.

A thin-film flip-chip III-nitride device may be formed by first growingan n-type region, a light emitting or active region, and a p-type regionon a growth substrate, such as sapphire, SiC, or GaN. Portions of thep-type region and the light emitting region are etched to exposeportions of the underlying n-type region. Metal electrodes which may bereflective, (e.g., silver, aluminum, or an alloy) are then formed on theexposed n- and p-type regions. When the diode is forward biased, thelight emitting region emits light at a wavelength determined by thecomposition of the III-nitride active layer. Forming such LEDs is wellknown.

The semiconductor LED 12 is then mounted on a mount 10 as a flip chip.Mount 10, may be any suitable material such as, for example, ceramic,aluminum, or silicon. Mount 10 includes metal electrodes that aresoldered or ultrasonically welded to the metal electrodes on thesemiconductor structure via interconnects, which may be, for example,gold or solder. Interconnects may be omitted if the electrodesthemselves can be connected, for example by an ultrasonic weld or anyother suitable joint. The multiple metal layers between thesemiconductor layers 12 and mount 10, including electrodes on thesemiconductor, electrodes on the mount, and interconnects, are shown inFIG. 2 as structure 14. Mount 10 acts as a mechanical support, providesan electrical interface between the n- and p-electrodes on the LED chipand a power supply, and provides heat sinking. Suitable mounts are wellknown.

To reduce the thickness of the LED and to prevent light from beingabsorbed by the growth substrate, the growth substrate is removed by amethod suitable to the substrate, such as etching, chemical-mechanicalpolishing, or laser melting, where a laser heats the interface of theIII-nitride structure and growth substrate, melting a portion of theIII-nitride structure and releasing the substrate from the semiconductorstructure. In one embodiment, removal of the growth substrate isperformed after an array of LEDs are mounted on a mount wafer and priorto the LEDs/mounts being singulated (e.g., by sawing).

After the growth substrate is removed, in some embodiments the remainingIII-nitride structure is thinned and/or roughened or patterned, forexample with a photonic crystal. The photonic crystal may be designed tomaximize emission into large angles relative to a normal to a topsurface of the device, for example. In some embodiments the photoniccrystal is configured such that the >50% of energy is emitted atangles >45° relative to a normal to a top surface of the device. Thedevice may be covered with an encapsulating material. In someembodiments, the growth substrate remains a part of the device. Thegrowth substrate may be coated with a reflective coating, such that amajority of light is emitted into large angles relative to a normal to atop surface of the device. A wavelength converting material such as oneor more phosphors may be formed over the semiconductor structure.

A cavity 20 separates LED 12 from waveguide 6. The sides 18 and bottom16 of the cavity adjacent to LED 12 are reflective. The cavity 20 may befilled with transparent material such as, for example, silicone. Adichroic filter layer 22 is disposed between waveguide 6 and cavity 20.The dichroic filter layer 22 may be configured such that blue lightemitted by the LED 12 at small angles, such as ray 24, is reflected,while blue light emitted by the LED 12 at large angles, such as ray 26,is transmitted. Suitable dichroic filters are well known and availablefrom, for example, Ocean Optics, 830 Douglas Ave. Dunedin, Fla. 34698.

The device illustrated in FIG. 2 may be formed by first forming the thinfilm flip chip LED 12 mounted on mount 10. The reflective sidewalls 18and reflective bottom 16 of cavity 20 are then formed. For example, areflective material such as, for example, TiO2 may be disposed in amoldable material such as, for example, silicone, then molded on mount10 to form the reflective sidewalls 18 and bottom 16. Alternatively,sidewalls 18 and bottom 16 may be pre-fabricated of a rigid material,coated with a reflective material if the rigid material itself is notreflective, then positioned on mount 10. Cavity 20 is then filled with atransparent material. Dichroic filter layer 22 is then coated over thetransparent material in cavity 20.

FIG. 3 illustrates a second example of a light source 8 which is coupledto a bottom surface of waveguide 6. Semiconductor LED 12 may be aIII-nitride thin film flip chip connected by interconnects 14 to a mount10, as described above. As in the device of FIG. 2, a cavity is formedby reflective sidewalls 18. The portion of the bottom 16 of the cavitythat is not occupied by LED 12 is made reflective. A solid transparentmaterial 30 such as glass occupies the cavity formed by reflectivesidewalls 18 and bottom 16. A dichroic filter layer 22, which reflectslight 24 and transmits light 26, as described above, is disposed overtransparent material 30.

The device illustrated in FIG. 3 may be formed by first forming the thinfilm flip chip LED 12 mounted on mount 10, as described above.Separately, a transparent material 30 such as a glass plate is coatedwith dichroic filter layer 22 diced to the desired size, before or afterforming dichroic filter layer 22. Transparent material 30 is attached toLED 12, for example by gluing with transparent epoxy or silicone.Reflective sidewalls 18 and bottom 16 are then formed by coating thesides and bottom of transparent material 30 with, for example, areflective metal such as silver or aluminum, reflective paint, areflective coating, or a reflective material such as TiO2 disposed in abinder such as, for example, silicone. The sides of transparent material30 may be coated with a reflective material before transparent material30 is attached to LED 12. A vacuum may be used to draw reflectivematerial in a binder into the spaces between transparent material 30 andmount 10, or the binder may be selected to wick under transparentmaterial 30, in order to form reflective bottoms 16. In someembodiments, mount 10 is reflective.

In some embodiments, in the devices illustrated in FIGS. 2 and 3, LED 12may have a lateral extent, in the dimension illustrated, of betweenseveral hundred microns and one or two millimeters. The space filledwith transparent material may have a lateral extent, in the dimensionillustrated, of between for example 1.1 and 2 times the lateral extentof LED 12, and a height of between for example 0.5 and 1.5 times thelateral extent of LED 12. In one example, LED 12 is 1 mm long, mount 10is 2 mm long, and transparent material 30 is 1.5 mm long and 1 mm tall.

In some embodiments, the edge 6A of waveguide 6 is shaped to directlight toward the area of the waveguide underlying the surface to beilluminated, as illustrated in FIG. 4, which is a top view of a portionof the waveguide. Squares 8 illustrate the locations of light sources,which may be the light sources illustrated in FIGS. 2 and 3. Edge 6Aincludes multiple curved portions, which may be coated with a reflectivematerial or which may be shaped to cause total internal reflection oflight emitted by light sources 8 toward edge 6A. The edge 6A may bescalloped, as illustrated in FIG. 4, or may have another shape. Thelight is directed toward active viewing area 50 of the display. Shapingthe edges of waveguide 6 may reduce the amount of light lost toabsorption by the LEDs by directing light incident on the waveguide edgeaway from the LEDs instead of back toward the LEDs. Shaping the edges ofwaveguide 6 may also improve the uniformity of light in the activeviewing area 50, may reduce the number of LEDs required for a givendisplay performance, and may reduce the bezel height 52, which is thedistance between the edge of the waveguide and the edge of the activeviewing area 50.

Light sources 8 may be positioned at even intervals across the bottom ofwaveguide 6, only near the edge of waveguide 6, or in any otherconfiguration. In some embodiments, some light sources emit blue light,some emit green light, and some emit red light. The red, green, and bluelight combines in waveguide 6 to form white light. In some embodiments,each light source emits white light, for example by wavelengthconverting some light emitted by a blue-emitting LED such that thewavelength converted light and the blue light combine to form whitelight. For example, a yellow-emitting phosphor may be combined with ablue-emitting LED to form white light, or a red-emitting phosphor and agreen-emitting phosphor may be combined with a blue-emitting LED to formwhite light. Additional phosphors or other wavelength-convertingmaterials that emit light of other colors may be added to achieve adesired color point. The phosphors may be disposed directly on LED 12 ofFIGS. 2 and 3, or between dichroic filter layer 22 and transparentmaterial 20 or 30, or between dichroic filter layer 22 and waveguide 6.In some embodiments, one or more remote phosphors may be disposed overwaveguide 6 in the active viewing area 50 of the display, illustrated inFIG. 4.

In a lighting system as described above where light is provided byseveral light sources, performance may be measured by whether the designprovides sufficient illumination, mixing, and uniformity of the light.Embodiments of the invention may provide sufficient illumination,mixing, and uniformity with fewer light sources, as compared to lightingsystems that do not incorporate features of the embodiments. In someapplications, such as backlights for displays, it is desirable tominimize the thickness of lighting system. Embodiments of the inventionmay provide the same performance in a thinner lighting system, ascompared to lighting systems that do not incorporate features of theembodiments.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

What is being claimed is:
 1. A device comprising: a waveguide comprisinga first section of transparent material; a light source disposedproximate a bottom surface of the waveguide, the light sourcecomprising: a mount upon which at least one semiconductor light emittingdiode is situated; sidewalls extending from the mount to the waveguideto form a cavity in which the semiconductor light emitting device issituated; and a second section of transparent material disposed withinthe cavity; wherein the sidewalls are reflective.
 2. The device of claim1 further comprising a surface to be illuminated disposed proximate atop surface of the waveguide.
 3. The device of claim 1 wherein areflective material is disposed on the sidewalls of the second sectionof transparent material.
 4. The device of claim 3 wherein the reflectivematerial comprises TiO2 disposed in a transparent binder material. 5.The device of claim 1 wherein: the semiconductor light emitting diode isaligned with a first portion of the bottom of the second section oftransparent material; and a second portion of the bottom of the secondsection of transparent material adjacent the first portion isreflective.
 6. The device of claim 1 wherein the second section oftransparent material is one of glass and silicone.
 7. The device ofclaim 1 wherein an edge of the waveguide is shaped to direct lighttoward a portion of a top surface of the waveguide.
 8. The device ofclaim 1 wherein an edge of the waveguide is curved.
 9. The device ofclaim 1 wherein an edge of the waveguide is shaped to cause totalinternal reflection of light incident on the edge.
 10. The device ofclaim 1 further comprising a dichroic filter disposed between thewaveguide and the second section of transparent material.
 11. The deviceof claim 1 further comprising a wavelength converting material disposedbetween the waveguide and the semiconductor light emitting diode. 12.The device of claim 1 wherein the second section of transparent materialhas a length greater than a length of the semiconductor light emittingdiode.
 13. The device of claim 1 wherein the semiconductor lightemitting diode comprises a photonic crystal configured such thatthe >50% of energy emitted by the semiconductor light emitting diode isemitted at angles >45° relative to a normal to a top surface of thesemiconductor light emitting diode.
 14. The device of claim 1 whereinthe semiconductor light emitting diode comprises: a sapphire growthsubstrate; and a reflective coating disposed on the sapphire growthsubstrate.
 15. A method comprising: attaching a semiconductor lightemitting diode to a mount; disposing reflective sidewalls on the mount,the reflective sidewalls forming a cavity in which the semiconductorlight emitting diode is disposed; filling the cavity with a firsttransparent material; and disposing the cavity containing thesemiconductor light emitting diode proximate a bottom surface of awaveguide comprising a second transparent material.
 16. The method ofclaim 15 wherein disposing reflective sidewalls on the mount comprisesmolding a reflective material comprising silicone on the mount.
 17. Themethod of claim 15 wherein filling the cavity with a first transparentmaterial comprises gluing a glass plate to the semiconductor lightemitting diode.
 18. The method of claim 17 wherein disposing reflectivesidewalls on the mount comprises coating sides and a bottom of the glassplate with a reflective coating.